The laser beam provides a means of writing, bar coding and decorative marking of plastics. This technique is advantageous over current printing technologies because of the ease at which the layout can be adjusted using graphic computer programs and also integrated into the production line. Laser marking enables a contract-free procedure even on soft, irregular surfaces that are not readily accessible. In addition it is ink-free which makes it long-lasting and solvent-free and, thus, more friendly to the environment. Speeds up to 10,000 mm/sec are possible with a CO.sub.2 laser while Nd-YAG laser allows up to 2000 mm/sec. Two-color injection molding offers marking results with the best contrast but the method is not flexible to change of layout. Thus, laser marking offers a versatile solution to most printing needs.
There are several laser types available for marking plastic surfaces. The Excimer laser with the frequency in the range of 196-351 nm leads to the marking of plastic surfaces by photochemical ablation or reaction. The Nd-YAG laser at lower power levels at 532 nm marks by leaching or selective bleaching of dyes and pigments. The YAG laser at 1064 nm marks by sublimation, discoloration, foaming or engraving. The CO.sub.2 laser at 10600 nm marks by thermochemical reaction, melting, vaporizing and engraving.
With carbon black as an ingredient, the laser creates lightly colored areas in contrast to a dark background. Carbon black decomposes into volatile components after absorbing the laser light. These volatile components foam the surface to scatter light and leave a lightly colored impression.
Contrast enhancing agents are descried in the prior art. EP 0 675 001 to Kato describes the use of zinc borate as a contrast enhancing additive where water of hydration is released. U.S. Pat. No. 4,595,647 to Spanjer describes a laser markable material useful for encapsulation of electronic devices. The material includes TiO.sub.2 or TiO.sub.2 +CrO.sub.3 as an additive to common plastic encapsulants which are formed from a mixture of resin, filler, carbon black, and mold release agent. When irradiated by a CO.sub.2 laser, the originally grey material forms a bright gold high contrast and durable mark. Desirable concentrations are described, in weight percent of the compound, as 1-5% TiO.sub.2 and 0-3% CrO.sub.3, with 1-3% TiO.sub.2 and 0.5-2% CrO.sub.3 being preferred. Carbon black is described as optional but a concentration in the range 0.1-3% by weight is desirable with 0.5-1% preferred.
GB 2,107,322 describes marking polymers which have low absorption in the infra-red range of wavelengths using a CO2 laser beam. The silicate additive having a high absorption at the wavelength 10.6 .mu. is calcium silicate, which in the form of wollastonite has an absorption of 96% at a wavelength of 10.6 .mu.. Other silicates described include aluminum silicates, e.g. in the form of China clay. The resin material described refer to polyolefins, polystyrene, and acrylonitrile-butadiene-styrene type materials.
Other references to silicates are found in EP 0 669 365 which describes the addition of silicates to polyolefins to yield a dark brown or black marking in the laser struck areas. Similarly, EP 0 111 357 uses metal silicates to obtain black markings on articles having a polyolefin surface. T. Kilp, "Laser marking of Plastics", Annu. Tech. Conf. Soc. Plast. Eng, (1991), 49th, 1901-1903, describes the effects of different silicates on the laser marking of polyolefins. Kaolin gave white marks on colored substrates while black marks were obtained when mica or titanium dioxide were incorporated into the substrate.
It is desirable to make further improvements in laser marking materials of the polyester type. In particular, a desired color combination is a dark background color and a light contrast color in the laser treated areas.